Process for making 2,5-dichlorothiophene



Patented Dec. 27, 1949 PROCESS FOR MAKING 2,5-DICHLORO- THIOPHENE GeorgeC. Johnson, Woodbury, N. J assignor to Socony-Vacuum Oil Company,Incorporated, a

corporation of New York No Drawing. Application May 29, 1947, Serial No.751,463

6' Claims.

This invention relates to an improved method for preparing2,5dichlorothiophene.

This compound has heretofore been prepared by the direct chlorination ofthiophene at room temperature and subsequent isolation thereof from theresulting crude thiophene-chlorine reaction mixture. The direct reactionbetween thiophene and chlorine proceeds easily and rapidly under normalconditions to yield a mixture comprising unreacted thiophene,monochlorothiophene, dichlorothiophenes, trichlorothiophenes,tetrachlorothiophene and chlorine addition products of thiophene.

To obtain a separation of the various chlorothiophenes, the methoddeveloped over sixty years ago and accredited to Victor Meyer hasheretofore generally been employed. This method involves heating thecrude chlorination reaction mixture with potassium hydroxide for severalhours to destroy addition products formed during the course of reaction,followed by addition of water and subsequent steam distillation andfractionation of the mixture 50 obtained to yield the variouschloropthiophenes. Subsequent modifications of this procedure haveinvolved removalof excess alkali and alkali chloride present after theheating period either by decantation, filtration, or water-washing anddistillation of the remaining liquid product containing chlorothiopheneseither at atmospheric or reduced pressure.

The dichlorothiophene fraction obtained as a result of the aboveprocedures consists of 2,5- dichlorothiophene, 2,3-dichlorothiophene,2,4-dichlorothiophene, a n d 3,4 dichlorothiophene. Since thesecompounds have fairly close boiling points, the isolation of2,5-dichlorothiophene from the other dichlorothiophenes, which arepresent in amounts up to 30 to 40 per cent by volume, is accomplishedonly by extremely efflcient fractional distillation. The probleminvolved in separating substantially pure 2,5-dichlorothiophene from theother dichlorothiophenes may be more fully realized by an examination ofthe boiling points of these compounds as set forth below:

Compound: Boiling point C. 2,5-dichlorothiophene 162.12,4-dichlorothiophene 167.6 2,3-dichlorothiophene 172.73,4-dichlorothiophene 182.0

It will thus be seen that 2,5-dichlorothiophene, 2 ,4-dich1orothiopheneand 2,3-dichlorothiophene have boiling point differences of the order of5 (3.- In order to efiectively separate pure 2.5-dichlorothiophene inadmixture with the others, repeated fractionations have generally beenfound to be necessary. By very efficient fractional distillation, forexample, with a -95 theoretical plate distillation column, the reactionmixture of dichlorothiophenes has been resolved into its components.However, such a separation step is accomplished, as will be realized,only at, considerable expense. Fractionation as a means for resolvingthe dichlorothiophene fraction is hence not at all feasible for theefiicient production of 2,5-dichlorothiophene in commercial quantities.

In accordance with the present invention, there has now been discovereda method of preparing- 2,5-dichlorothiophene which tends to overcome thedifficulties existent in the process heretofore employed. By carryingout the process of this invention, it has been found that thedichlorothiophene fraction is inherently substantially pure2,5-dichlorothiophene and that no distillation difficulties attend itsseparation. Broadly stated, the process contemplated herein comprisesreacting 2-chlorothiophene with chlorine, heating the resulting reactionproduct mixture in the presence of alkali to decompose the chlorineaddition products formed and distilling the resultant mixture to obtaina dichlorothiophene fraction consisting essentially of pure2,5-dichlorothiophene.

It has been found by contacting z-chlorothiophene with chlorine,preferably in slight molar excess and then heating the product mixturewith alkali and distilling, that a comparatively simple,- inexpensivemethod for synthesizing 2,5-dichlorothiophene is provided. Moreover, theprocess of this invention permits higher yields of 2,5-dichlorothiopheneper mole of thiophene initially employed in preparing the2-chlorothiophene than the methods of the prior art. This increasedyield in product causes a substantial reduction in expenditure requiredper unit of 2,5-dichlorothiophene obtained and hence gives rise to ahighly economical and efficient method of production.

It is accordingly an object of the present invention to provide aneffective process for preparing 2,5-dichlorothiophene wherein thedistillation difiiculties prevalent in the prior art procedure forseparation of the dichloropthiophene fraction resulting uponchlorination of thiophene have been overcome. A further object is toafford a process for obtaining essentially pure 2,5-dichlorothiophene. Astill further object is the provision of a comparatively simpleeconomical method for making 2,5-dichlorothiophene, which requires lessapparatus and attention than has been necessary in the prior artprocedure.

These and other objects which will be recognized by those skilled in theart are attained in accordance with the present invention wherein2,5-dichlorothiophene is prepared by contacting 2-chlorothiophene withchlorine and thereafter maintaining'the "resulting reaction mixture atan elevated temperaturein the presence of alkali to decomposesubstantially all of the addition products resulting during theaforesaid chlorination.

Fractionation of the mixture so Obtained yields substantially pure2,5-dichlorothiophene. Infra-- red absorption measurements haveindicated that the dichlorothiophene fraction is composed off 98-100 percent 2,5-dichlorothiophene and 2 per cent 2,3-dichlorothiophene withouteven the merest trace of 2,4-dichlorothiophene 0x334411-chlorothiophene.

The 2-chlorothiophene reactant employed in the present process may beobtained from "any source. Generally, however, the2-chlorotl'iioph'e'n'e is prepared by the reaction of thi'ophene withchlorine gas, treatment of the erude product with alkali to decomposechlorine additionproducts' and finall'ydistillation to -recover-2-lil'ordthiophene. "This 1 distillation-is readily performed, since2-chlorothiophene may be separ'ated easily from i any unreactedthiophene (442 C.'-difference in-boiling point) and frOm any -Zj-'dichlorothiophene formed -('33.8 C. difieren'ce in boiling point)Furthermore, the latter 'sep'a'ration need-be ma'de with no particularsharpness as' th'e inclusion-of --2-,5-dichlorothiophene in the 2hlorothiophene will -do no harm. The monoclilorothiophene obtainedby theabove' procedure is slib's'tantia'lly pure -2-ohlorothiophe'ne with "nomoreth'an a few tenths of one percerit-of-3-chlorothiophene, *the onlyother possible monochlo'ro derivative. -As analternatemethod of preparin2 cl'ilcr'othiophene, the alkali treatment of "the crud'e'reac'tionproduct may be omit'ted-and the proHuctdire'ctl-y distilled undervacuun'i to-obtai'n' a out rich-in 2-chlorothiophene.

"The chlorination -of -2-chlorothiophene in accordance with the presentinvention will 'generally be ca'rried out" in -the -temp eraturerange-of fr'om about C. to -about 150 C. As the chlorinatienreaction"-'proceeds-by the addition'ofgaseous chlorine to -tliemonochlor'othiophene, the temperatu'r'e o'f -th"e r'eactionmixturerises. It has, 'however, -been found desirable to #r'nai'ntainth'e temperature of thereaction mixture below about 50 8. duringchlo'rination. This can -be effectively -'-accomplished by water-cooling"the vessel in which chl orination is taking place-or by- 'othersuitable coolin'g means known to the-art. Grdin'arily, chlorine will bebubbled'into2- 'chlorothi'ophene at a "rate which will insure thoroughmixing and contact of the two reactants. While an equimolar ratio of2-chlorothiophene to 01110- rinelean be employedwin bringing aboutthedeslred -results, it is desirable to use a slight molar excess oichlorine per mole of 2-chlorothiophene under -treatment.

At the completion of the chlorination reaction, the resulting mixture isbrought into 'contact with alkali and the mixture is maintained at anelevated temperature for a period sufficien't to decompose the: chlorineaddition products produced during the course of chlorination. Thetemperatureduring this period will generally be between about 90 CJ-and:about 200 I C. and preferably-"between "about 7 100 C. and about 'C.--'I he optimum heating period -may be readily --stablished for a -particular degree of chlorination; Under the usual conditions contemplatedby the process of this invention, the period of heating will ordinarilylie between about 1 hour and about 24 hours. The time of heating of thechlorinated mixture may be reduced by passing a stream of 5 an inert gasthrough the mixture during the heating period. Any'of thecommonly-employed alkaline materials may be used for aiding in the decompositionof the chlorine addition products in the reaction mixture. Generally, anoxide, hydroxide-or carbonate of the alkali or alkaline eaif-th metalswillbe employed, preferably sodium hydroxide, potassium hydroxide or amixture *of tne two.

whe'n' theheating period is completed, solid 5 matter "present=c'onsis'ting essentially of excess alkali-and-salts :thereof is removedby filtration, d'ecantation, water-washing, or other suitable means. Themixture is then fractionated to yield a dililorothiophene fractionconsisting of substantially pure 2,5-dichlorothiophene. As an alternateprocess the m'ixture :may te ste'amdistilled without 'prior re'm'ovalofs'alt and eiz'c'ess alkali.

The iollowing example illus'tratiye df the results -contemplated by thepresent invention:

'Eirample I Pure 2-chlorothiophene -(B. P. 28. 1, nn:- 1.5487) waschlorinate'd at =50 C. with e. slightmolar excess of chlo'rine at a rateof "about 18 liters ofchl'orine per hour ip'er igram moleof zchlorothi'ophene. The temperatureuwas maintained at 50 C. by means ofan:ice bath. Addition .lp'roducts 'were decomposed by heating thereaction ;product for I about 24; :hours Y at 400-125 C. with-0.5 moleof sodium' hydroxideiand 0,5 'mole of potassium hydroxide per mole of'chlorinated thiophene. 'After filtering ofi the-excess alkali andsalts, =the dichlorothiophene:fraction was distilled. A large fractionof 2g5-dichlorothiophene ('B. :P. 162.1, "nD &1.5626) was :obtained. Asharp 'rise in temperature' occurredsfrom 162;1 t0 198.7, the boiling:ipoint :of 2;3,5-'trichlorothiophene. This indicates th'at there was:no substantial amount of other dichlorothiophenes present. The contentof the 2g5 dicl1lorothiophene was foun'd by infra-red absorptionmeasurem'ents 'to bein exces's' o'f 98 :per bentby' vcliime of:thediehlorothiophene naction.

From th'e above :it will be se'en that the dinchlorothiophene fiactionobtained in accordance with the pr oeess of this invention is-siibst'antially pure 2 ,5-dichlorothiophene and that the same isreadily obtained without the distillation clifficiilties inherent in theprior art procedure wherein -dichlorothiophenes "other than216-dichlorothiophen'e "w'l e present in amounts of the order. of 40 ercent b v volume, as ishown by the H following 'exam ple, illustrative*of the previously em ployed process entailing adirect chlorinationzofthiophene.

Example II -'I:hiophene -was:chlorinated at' aC. withone mole ofchlorine per mole of thiophen'e. The temperature was maintained at 50 C.by an ice bath. The crude reaction, product Was heatedioraboutlshoursat*aboiitloociwithamixture of e'qu'al 'partsbyweight' 6fsodium hydroxide and otassium hydroxide equivalent to 0.2 mole of alkaliper mole of 'thiophne. "Salts and unreaeted alkanwereremoved byflltratio'nfiand tlie filtered product was distilled. IHe iistillationfractions-mere an'alyzed 'by boiling poisits, by "re- 25 fractive indexnreasurements and by ir'iffia-reti absorption spectrograms. Thecomposition of the dichlorothiophene fraction was 2,5-dichlorothiophene,63.2 per cent by volume; other dichlorothiophenes, 36.8 per cent byvolume. 2,4- dichlorothiophene, 2,3-dichlorothiophene and 3,4-dichlorothiophene were each present in substantial amounts.

I claim:

1. A process for preparing substantially pure 2,5-dichlorothiophene,comprising reacting a reaction mass consisting of chlorine and 2-chlorothiophene, heating the resulting reaction product mixture in thepresence of alkali to decompose chlorine addition products formedtherein as a result of the aforesaid reaction and distilling the mixtureso obtained to yield a dichlorothiophene fraction consisting essentiallyof 2,5-dichlorothiophene.

2. A process for preparing substantially pure 2,5-dichlorothiophene,comprising reacting a reaction mass consisting of chlorine and 2-chlorothiophene, heating the resulting reaction product mixture in thepresence of alkali to decompose chlorine addition products formedtherein as a result of the aforesaid reaction, removing solid matterfrom the resulting solution and distilling the same to yield adichlorothiophene fraction consisting essentially of 2,5 dichlorothiophene.

3. A process for preparing substantially pure 2,5-dichlorothiophene,comprising reacting a reaction mass consisting of chlorine and 2-chlorothiophene, heating the resulting reaction product mixture at atemperature between about 100 C. and about 125 C. in the presence ofalkali to decompose chlorine addition products formed therein as aresult of the aforesaid reaction and distilling the mixture so obtainedto yield a dichlorothiophene fraction consisting essentially of2,5-dichlorothiophene.

4. A process for preparing substantially pure 2,5-dichrothiophene,comprising reacting chlorine with substantially pure 2-chlorothiophene,maintaining the temperature of the reaction mixture below about 50 C.during chlorination, heating the resulting reaction product mixture inthe presence of alkali to decompose chlorine addition products formedtherein as a result of the aforesaid reaction and distilling the mixtureso obtained to yield a diohlorothiophene fraction consisting essentiallyof 2,5-dichlorothiophene.

5. A process for preparing substantially pure 2,5-dichlorothiophene,comprising reacting chlorine with substantially pure 2-chlorothiophene,maintaining the temperature of the reaction mix ture below about C.during chlorination, heating the resulting reaction product mixture at atemperature between about C. and about C. in the presence of alkali todecompose chlorine addition products formed therein as a result of theaforesaid reaction and distilling the mixture so obtained to yield adichlorothiophene fraction consisting essentially of2,5-dichlorothiophene.

6. A process for preparing substantially pure 2,5-dichlorothiophene,comprising reacting substantially pure 2-chlorothiophene with an excessof chlorine, maintaining the temperature of the reaction mixture belowabout 50 C. during chlorination, heating the resulting reaction productmixture at a temperature between about 100 C. and about 125 C. in thepresence of an alkali metal hydroxide to decompose chlorine additionproducts formed therein as a result of the aforesaid reaction, removingsolid matter from the resulting solution and distilling the same toyield a dichlorothiophene fraction consisting essentially of2,5-dichlorothiophene.

GEORGE C. JOHNSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

Ber. 17, 794-5 (1884) Ber. 19, 650 (1886).

Coonradt, J. Am. Chem. Soc. 70, 2565 (1948).

Weygand, Organic Preparations," pages 51 and 71, IntersciencePublishers, 1945.

